A.I. Explanation:

First sentence: Why this is the first line? This paragraph is the first sentence because it introduces the main topic of building a bookshelf. It gives a clear starting point for the task. It does not use any pronouns or definite articles that refer to previous ideas. Why other lines are not chosen? Paragraph 2 starts with 'Select', which is not independent because it needs context. Paragraph 3 starts with 'Use', which also needs context. Paragraph 4 starts with 'Once assembled', which refers to something that is not yet mentioned.

Concluding sentence: Why this is the last line? This paragraph is the last sentence because it wraps up the process of building a bookshelf. It gives the final steps after assembly, showing the completion of the task.

Paragraph pairs:

• Pair 1 (Paragraph 1 → Paragraph 2): Paragraph 2 connects with Paragraph 1 because it continues the process of building a bookshelf. The first paragraph talks about measuring the space, and the second paragraph talks about selecting wood, which is the next step.
• Pair 2 (Paragraph 2 → Paragraph 3): Paragraph 3 connects with Paragraph 2 because it follows the selection of wood. After choosing the wood, the next step is to cut it and assemble it, which is what Paragraph 3 explains.
• Pair 3 (Paragraph 3 → Paragraph 4): Paragraph 4 connects with Paragraph 3 because it describes what to do after assembling the bookshelf. After cutting and assembling, the next step is to apply the finish and fill the bookshelf.